pcolor and patch

[John W. Eaton]

On 24-Feb-2010, RENOO wrote:

| This is my first post here as I am starting using Octave and I must confess
| that I am very happy of the quality of this software so far:clap:
| 
| 
| However, there is one feature I can not make.
| 
| I would like to plot my data using a pcolor and then add a patch to hide a
| part of those data. 
| I assumed that using the contourf command is equivalent at finding the
| cordinates of the isoline with the contourc command and then applying the
| patch command.
| 
| So here is a simple example of my script : 
| 
| [X,Y,Z] = peaks(50);
| pcolor(X,Y,Z); shading flat
| hold on
| contourf(X,Y,Z,[1 1],'k')
| 
| error: patch: property value is missing
| error: evaluating if command near line 205, column 3
| error: called from `contourf:parse_args' in file
| `/usr/share/octave/3.0.5/m/plot/contourf.m'                                   
                                             
| error: called from `contourf' in file
| `/usr/share/octave/3.0.5/m/plot/contourf.m'    
| 
| So I tried without any properties : 
| octave:14> contourf(X,Y,Z,[1 1])                                              
    
| error: gnuplot (as of v4.2) only supports 2D filled patches
| error: evaluating if command near line 537, column 9
| ....
| 
| google ( and the message error ) told me it comes from gnuplot backend. 
| 
| So I tried from this  http://old.nabble.com/FLTK-howto--td27703703.html post 
| 
| backend("fltk") 
| 
| and this leads me to the following error message : 
| 
| 
| octave3.2:3> [X,Y,Z] = peaks(50);
| octave3.2:4> pcolor(X,Y,Z); shading flat
| octave3.2:5> hold on
| octave3.2:6> contourf(X,Y,Z,[1 1],'k')
| error: mx_el_lt: nonconformant arguments (op1 is 1x2, op2 is 0x0)
| error: evaluating argument list element number 1
| error: evaluating argument list element number 1
| error: called from:
| error:   /usr/share/octave/3.2.2/m/plot/__contour__.m at line 238, column 15
| error:   /usr/share/octave/3.2.2/m/plot/__contour__.m at line 188, column 3
| error:   /usr/share/octave/3.2.2/m/plot/contourf.m at line 73, column 5
| 
| I am running octave 
| octave3.2:6> version
| ans = 3.2.2
| 
| and I also tried with QtOctave version 0.8.1 which runs octave 
| octave:13> version
| ans = 3.0.5
| 
| but in vain.
| 
| Any help  ?

You can avoid the nonconformant argument error by setting the level
argument to contourf to be something like [1, 1+eps] instead of [1, 1].
That's a bug, and I've checked in the following change:

  http://hg.savannah.gnu.org/hgweb/octave/rev/f074aa6b2c80

But I don't think gnuplot 4.2 will work for this plot even with this patch:

  error: gnuplot (as of v4.2) only supports 3D filled triangular patches
  error: called from:
  error:   /home/jwe/src/octave/scripts/plot/__go_draw_axes__.m at line 592, 
column 6
  error:   /home/jwe/src/octave/scripts/plot/__go_draw_figure__.m at line 102, 
column 1
  error:   /home/jwe/src/octave/scripts/plot/gnuplot_drawnow.m at line 99, 
column 5

I don't know whether later versions avoid this problem.

Using the fltk backend with the current sources, I get the first plot
attached below.  I suspect that is not what you are looking for.
Using contour instead of contourf, I get the second attached plot.  It
looks more reasonable, but I'm not sure whether that is the image you
want.

BTW, it is best to report bugs to the address@hidden mailing list.

jwe